1. Field of the Invention
The present invention relates to an illumination unit capable of increasing the efficiency of collecting light emitted from a light source and an image projection apparatus employing the illumination unit.
2. Description of the Related Art
In general, illumination units include a light source emitting light, and an illumination optical system transmitting light emitted from the light source. Illumination units are widely used for image projection apparatuses which create an image using an image-forming device such as a liquid crystal display (LCD) device or a digital light processing panel (DLP) comprised of a two dimensional array of micromirrors.
A metal halide lamp or a super-high voltage mercury lamp has been used as a light source of an illumination unit. Since the life span of these lamps is several thousands hours at most, the lamps need to be frequently replaced. To solve this problem, research on the use of a compact light-emitting device such as a light-emitting diode (LED) having a relatively longer life span has been conducted. Since an LED radiates light divergently, an illumination unit needs to collect and collimate the light emitted from the LED such that the light can propagate in one direction.
The LED emits relatively less light than the metal halide lamp or the super-high voltage mercury lamp. Accordingly, an array of LED modules must be used as a light source of image projection apparatuses.
To collimate light emitted from an LED, the LED modules generally comprise lenses. The array of LED modules using general lenses has low light efficiency as explained below.
The product of the emission area and the solid angle of the light emitted by the LED is a conserved value called the “etendue”. Since the etendue is conserved, the product of the emission area and the solid angle of the light emitted by the LED should be equal to the product of the area of the image-forming device and the solid angle of incidence of the image-forming device. The etendue of the image-forming device is determined geometrically.
When an array of LED modules is used, the emission area of the array of LED modules is larger than the emission area of one LED module because the emission area increases in proportion to the number of LED modules.
Here, the solid angle of emission of each LED module is identical to the solid angle of emission of the array of LED modules, and the area of the image-forming device is fixed. According to etendue conservation, the solid angle of the incidence of the image-forming device is greater when the array of LED modules is used than when one LED module is used. Accordingly, some light exists outside the range of solid angle where light can be effectively projected by a projection lens and loss of light occurs, thereby reducing light efficiency. As a result, the luminance of the image-forming device is limited in spite of a greater number of LEDs.